1. Field of the Invention
This invention relates to both a line material adapted for forming electronic circuits and an electronic device such as an active matrix type liquid crystal display using such line material.
2. Description of the Related Art
Active matrix type liquid crystal display elements are gaining attention, which display element uses thin film transistors (TFT) made of an amorphous silicon (a-Si) film as its switching elements. This is because the feature that the a-Si film can be formed at low temperatures on an inexpensive glass substrate may open the way to producing a large area, high density and high definition panel display (flat-type television) inexpensively. This type of active matrix type liquid crystal display element is made up of some hundreds to thousands of address and data lines with each of their intersecting points provided with a TFT, a pixel, and a storage capacitor. The number of the TFTs or the storage capacitors is so large (some ten thousands to some millions) that the performance of this type of display element is governed by their yield.
While each point of intersection between the address and data lines and their inter-line portion at each TFT section are insulated by an insulating film, the presence of pin holes in this insulating film causes two line defects on the address and data lines running through this section. Similarly, any pin holes present in the insulating film between each storage capacitor line and each pixel electrode produce point defects. The insulating films are formed by a plasma CVD (chemical vapor deposition) method, which can form films at low temperatures. However, this plasma CVD method could not eliminate the pin holes caused by contaminants adhering to the chamber walls.
A solution to preventing the generation of such defects is an anodized film. Anodized films have pin holes removed by itself with the application of an electric field thereto during their formation process, and thus the pin holes are eliminated. Anodizable metals include Ta and Mo-Ta alloys. Oxide films of Ta are not satisfactory in resistivity, while oxide films of Mo-Ta alloys are poor in thermal resistance, both missing high thermal resistance and high resistivity requirements for the material of the address and data lines.
Although small-size display elements require only high insulation resistance for their oxide films, the active matrix type liquid crystal display element having a large display area with smallest possible pixels must have thin and long signal lines to each TFT, i.e., the gate line and the data line, and this naturally increases their line resistance. On the other hand, the line resistance must be minimized to eliminate the waveform distortion and delay of pulse signals, thereby requiring low resistivity of line forming metals. In addition, the reverse-staggered TFT structure that a gate electrode line and an a-Si film are sequentially formed and deposited on a glass substrate requires not only a thin gate electrode line but also the gate electrode line being resistant to chemical treatments to which it must undergo in succeeding processes.
The data electrode line materials satisfying these requirements include Ta and Ti. However, their resistivity is still too high to achieve a larger display area and a higher density. It is why the development of a low resistance, highly formable, and chemical treatment resistant material is called for. The staggered TFT structure that the source and drain electrode lines are arranged on the substrate side likewise requires a high resistance, highly formable and chemically resistant material. This problem must be overcome also for non-TFT based active matrix type display elements.
Solutions to the above problem are disclosed in Japanese Patent Examined Publications Nos. 48910/86 and 65669/1988: the Mo-Ta alloy and the Ta film on a cubic metal such as shown in FIG. 1, respectively. While these films are proposed as a patterned film satisfying the requirements of low resistance and anodization, the anodized films of Mo-Ta alloys and cubic metals (excluding Ta) are not satisfactory in thermal resistance. The anodized films formed of the above described metals exhibit satisfactory insulation immediately after their formation but such insulation gets deteriorated upon being subjected to thermal treatments of 200.degree. C. or more. The reason is assumed to be that the anodic oxides of cubic metals and Mo-Ta alloys forming the lower layer of the laminated film deteriorate after the thermal treatment in the anodization process.
An object of the invention is to provide a line material which can easily be formed into a low resistance.
Another object of the invention is to provide a line material which can easily be formed into a low resistance and high quality insulating film by an anodization process.
Still another object of the invention is to provide an electronic device having a line made of a low resistance and high quality insulating film which has easily been formed by an oxidation process.
Still another object of the invention is to provide a liquid crystal display which can be easily constructed and whose performance is highly reliable.